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Nanoparticulate inorganic UV absorbers: a review.

Abstract Inorganic nanoparticles with UV-absorbing properties are an important class of UV filters. They can be used in various applications and in a variety of forms, including suspensions, nanocomposites, and solid thin films. In this review, an overview of the synthetic methods and their respective products is given for the most popular UV-absorbing nanomaterials, including zinc oxide, titanium dioxide, cerium dioxide and ferrous oxides, and oxyhydroxides.

Keywords Nanoparticles, UV filters, ZnO, Ti[O.sub.2], Ce[O.sub.2], [Fe.sub.2][O.sub.3], FeO(OH)

Introduction

Ultraviolet irradiation is a natural part of the light emission spectrum of the sun. With its lower wavelength (UVA: 400-320 nm, UVB: 320-230 nm) and higher energy (over 3.2 eV per photon) than the visible part of the spectrum, it is known to cause a multitude of problems. In technical applications, UV irradiation can cause decomposition of polymers and coatings, affecting both mechanical properties and color. (1) Inks and colors on works of art can be degraded and artifacts of historical importance damaged through extensive illumination (as would be required when showcasing these in a museum). This sort of damage is quantified by the Krochmann damage function. (2,3) In order to reduce the impact of the irradiation, additional protective coating is required. (4-6)

Living organisms including humans can also be damaged through excessive exposure to sunlight with both short-term symptoms like sunburn and long-term hazards like skin cancer, (7,8) making proper UV protection indispensable.

Therefore, materials with UV-blocking capabilities are important for a wide range of applications, from sun lotions as skin protection to optics (filters for photography, coatings for reading glasses). There are two general types of UV-protecting agents: organic and inorganic. Organic UV filters are molecules with conjugated [pi]-electron systems, capable of absorbing UV irradiation such as various derivates of methoxycinnamate. The molecules, however, are prone to direct photodegradation (9) or decompose when subjected to radical products of the disintegration of the coating matrix (10,11) and thus offer a limited protection time.

Inorganic UV blockers are various metal oxides. The fundamental effect for UV absorption is the convenient band gap in the 3.0-3.4 eV range that corresponds with wavelengths below 400 nm, though scattering and reflection by the particles also contribute to the protecting effect. The material can be applied as thin films or as dispersed particles. The active compounds are not decomposed by irradiation and are insoluble at neutral pH, thus being suitable for applications with lasting UV protection. Even for short-term UV protection, for example, as sunscreen formulations, inorganic, nanoparticulate UV filters have certain advantages. Their dispersions are less greasy and feel more pleasant to the touch. Nanoparticles can further expand the range of applications, with particles below 20 nm in diameter being unable to scatter light in the visible range but retaining their UV-shielding properties. These materials would allow for optically clear composites that still offer effective protection from UV irradiation. In this article, a review of the various types of nanoparticles with UV-blocking capabilities and their possible synthesis methods shall be given.

Current applications of nanoparticles in coatings and films

Nanoparticles of ZnO, Ti[O.sub.2], Ce[O.sub.2], [Fe.sub.2][O.sub.3], and FeO(OH) are established components for a multitude of functional coatings. A comprehensive summary of employed matrix materials and applications would each require separate reviews, as various possibilities have become available with the advancement of nanotechnology. Mainly, these coatings have either a protective or an optical function. One example of optical applications is the PMMA coating containing Ti[O.sub.2] nanoparticles with superior antireflective properties12 or the inclusion of nano-ceria to improve the UV stability of the coating. (13) Coatings with a focus on mechanical properties are often used to protect surfaces such as touch screens from scratching. Embedded nanoparticles are typically harder than the coating matrix and provide better abrasive resistance, such as scratch resistant polycarbonate with ZnO nanoparticles. (14)

Other coatings provide a chemical function, such as anticorrosion epoxy coatings with [Fe.sub.2][O.sub.3]. (15) These composite coatings are important for the metal components in reinforced concrete. Not only do they prevent oxygen and water from reaching the metal, they are also mechanically more stable, thus further extending the life expectancy for the metal components.

The true potential of nanocomposite systems comes to bear when multiple functions are achieved at once. ZnO and Ti[O.sub.2] nanoparticles can either be directly coated onto fibers such as cotton (16-20) or be processed into composite foils (21) for self-cleaning capabilities, antibacterial properties, and UV protection. Changes to wettability, reflectivity, and photocatalytic self-cleaning can be achieved with one composite coating. (22)

These nanoparticle-containing systems are ready to be used for widespread coating applications. Aloui et al. demonstrated the feasibility for simple process integration by combining commercial suspensions of nano-Ti[O.sub.2] and [Fe.sub.3][O.sub.4] with commercial clear coatings to create UV protection for wood. Currie and Van de Belt patented a latex suitable for various metal oxide nanoparticles depending on the application at hand. (23) Charpentier and Burgess patented a self-cleaning coating with titania nanoparticles. (24) Other companies developed nanocomposites for heat management, (25,26) heat-resistant dielectric components for plasma displays, (27) or corrosion-resisting hardcoats for demanding applications, such as the one patented by the EADS. (25) A rather unconventional nanocomposite alloy containing ceria and titania nanoparticles was patented by Feng et al. (29) It can be applied in a hot-dipping process and shows remarkable qualities protecting titanium alloys from corrosion and wear.

In all presented examples, the characteristics of the coating are dependent on all its constituents. To have reliable quality of the final product, the properties of the coating matrix and the nanoparticles both have to be controlled during synthesis and fabrication. This is most easily noticeable in the relation between light scattering and particle size, but other aspects such as particle morphology, modification, and crystallinity may be of equal importance to the properties of the coating. This review covers the various synthesis routes and their respective products for nanoparticles with UV-absorbing properties.

Nanoparticles as UV absorbers and their ecological and toxicological risks

The emergence of nanoparticles in common products such as sun lotions has led to the questions whether the unusually high-specific surface area and small particle size may lead to unsuspected spread in the environment, intake of the nanoparticles, and an increased toxicity. These are general concerns and universally encountered when working with any type of nanoparticles. With growing consciousness, more studies have been performed on the safety and stability of such compounds as nanoparticulate zinc oxide (zincite), titanium dioxide (titania), cerium dioxide (ceria), and various iron oxides and oxide-hydroxides. (30-37)

The growing knowledge and understanding of hazards emanating from nanoparticles is illustrated in the constant revision of the regulations over the last years. In the US, zinc oxide was approved by the FDA in 1999 for sun protection purposes. (38) In the European Union, a first review on safety aspects of zinc oxide as a possible UV filter was conducted by the Scientific Committee on Cosmetic Products and Non-Food Products intended for Customers (SCCNFP) in 200339 and came to the following conclusion:

While "in general, ZnO may be considered as a nontoxic substance, including when used in cosmetic products," the "lack of reliable data on the percutaneous absorption of micronised ZnO and the potential for absorption by inhalation has not been considered," which is the reason why zinc oxide was not a generally approved sunscreen component according to European law in 2009. (40)

Since then, many investigations have shown that zinc oxide is not harmful when ingested or applied to skin as well as not being sensitizing or irritating even to already sunburned skin. (31,39) A revision in 2012 approved zinc oxide as the UV-absorbing component in liquid lotions with concentrations of up to 25 wt%. (41)

While being safe for higher organisms, zinc oxide can pose a serious threat to bacteria due to its ability to distort bacterial cell membranes (42) and to form reactive oxygen species (ROS), (30) even when not illuminated by UV light. (32) This can be used for manufacturing nanoparticle-coated bandages with antimicrobial properties (19) but should be considered where a release in the environment can occur.

Similar to zincite, titania and ceria nanoparticles are known to cause the formation of reactive oxygen species when irradiated with UV light. While these ROS are known to damage DNA in vitro, living tissue is only endangered when the particles manage to pierce the cell membranes. As reviewed by Newman, Schindler et al., (33,34) this is usually not the case. Neither zinc oxide nor titanium dioxide nanoparticles readily penetrate the epidermis, thus making "... the public health benefits of sunscreens containing nano Ti[O.sub.2] and/or ZnO outweigh human safety concerns for these UV filters." Titania nanoparticles can cause damage to bacteria, (32) but the penetration capabilities vary, depending on bacterial strain and properties of the particles. (35) Ceria, on the other hand, does not produce reactive oxygen species under biological conditions, as reported by Xia et al. (30) Instead, ceria nanoparticles help living tissue to activate its protective mechanisms and reduce the oxidative stress caused by other sources.

The formation of nanoparticles of iron oxides and oxyhydroxides is a naturally occurring process. (36) Not only are these particles important for various microorganisms (the availability of iron ions being a limiting factor to growth in some environments) but especially the weakly crystalline oxyhydroxides are also important as natural scavengers for heavy metal ions. Under natural conditions, iron(III) nanoparticles can be considered as not harmful to the environment and can even mitigate water acidification to some degree. (43)

Still, nanoparticles can cause damage to higher organisms, especially when inhaled. (44-46) This is a common trait of nanoparticles and their interaction with the delicate and absorption-oriented cells of the pulmonary organs. The adverse effects are not necessarily related to the chemical reactivity and can occur with nonreactive particles such as silica particles. (47)

Fortunately, an exposition of the airways is rather unlikely, but can be a concern in terms of workplace safety, especially when production methods that rely on airborne nanoparticles (such as flame spray pyrolysis) are applied.

Synthesis methods for zinc oxide

Among the most common inorganic UV blockers is zinc oxide. With a band gap of 3.37 eV, (48) it is an efficient UV blocker for wavelengths below 360 nm (Fig. 1). Zinc oxide features a wurzite structure. A zinc blende modification can be achieved through epitactic growth (49) and very high pressure can form a rock-salt structure. (50)

There are various approaches to the synthesis of nanoparticles. Among the common methods for zinc oxide nanoparticles are mechanochemical, sol-gel, and hydrothermal routes.

Mechanochemical methods

Various mechanochemical routes have been described. In general terms, a zinc salt providing the zinc cations and another salt providing the oxygen species are mixed in a third salt, usually sodium chloride, which acts as a diluent. After milling the powder for a fixed amount of time, a new zinc salt like zinc oxide or zinc carbonate is formed. This precursor is then calcinated and reacts to zinc oxide particles. Various mechanochemical methods can be compared in Table 1.

Mechanochemical reactions have certain advantages. The starting materials are common, low-priced commodities. The reaction setup is simple, well scalable, and has industrial applications. (55) However, mechanochemical reactions have certain disadvantages. While they do not require liquid solvents, large amounts of the matrix salt (usually sodium chloride) are needed to prevent the formation of large aggregates and a significant amount of solid byproduct of low value like NaCl (51) or [K.sub.2]S[O.sub.4.sup.54] is formed during the reaction. The separation of solid powders is problematic, usually performed by dissolving and washing away the matrix salt along with any formed co-products. This provides large amounts of saline water which have to be disposed of.

Another disadvantage is the broad particle size distribution. Contrary to sol-gel methods, no controlling mechanism is limiting the particle size as the particles are formed and broken down purely by physical collision. For applications that require small nanoparticles with a narrow size distribution, the products of mechanochemical methods will hardly offer a satisfying solution since a good portion of the formed particles is larger than 100 nm and particle size varies wildly, the only exception to this being the method by Shen et al.

Finally, the calcinations step requires the particles to be heated to high temperatures. In order to prevent the particles from sintering, the matrix salt still has to be present at this point, thus providing additional heat capacity that invariably increases the energy requirements of the process.

Sol-gel methods

Sol-gel syntheses are the most common procedures for the formation of zinc oxide nanoparticles. A multitude of methods are published which share certain common principles. An alcohol-soluble zinc salt (usually zinc acetate or chloride) is dissolved in a low molecular weight alcohol (methanol, 2-propanol) and mixed with an alcoholic solution of a hydroxide source (lithium hydroxide, sodium hydroxide). The mixture can be thermally treated or aged for some amount of time to promote particle formation and growth. (58-60) Table 2 offers an overview of the published methods.

The most popular method is based on the synthesis published by Spanhel and Anderson, (62) which was used with minor variations (58,59,70) and continues to be further investigated to this day. Among the published improvements is the addition of organic compounds like soluble starch (64) or carboxylates and isocyanides. (70) Adsorption of the organic molecules to the nanoparticle surface limits the growth of the particles and allows for further size control. This surface coating may also be applied to improve compatibility of the particles with various organic solvents. (66) The small particle size and narrow distribution are the main advantages of the various sol-gel methods. The reactions do not require much in the way of heating and can be performed at ambient pressure and exposure to air. A calcination step is usually not necessary. The necessity for organic solvents is a minor disadvantage, since only alcohols with low boiling points are used. These solvents can be more easily removed and recycled than other common solvents like toluene or higher alkanes.

High-temperature methods

For the purpose of this article, all methods that require temperatures above 100[degrees]C for the formation of the particles are considered high-temperature methods, thus excluding reactions with a calcination step of already formed particles. For zinc oxide nanoparticles, this includes two main approaches: hydrothermal syntheses and hot injection or high-temperature decomposition reactions.

Hydrothermal reactions use high pressure to increase the boiling point of water well above 100[degrees]C, thus increasing solubility of various salts and providing an increased amount of thermal energy while still employing water as the reaction medium. Hot injection relies on very high-boiling organic solvents like long-chained alkanes, alkyl amines, phosphates, phosphonates, or ethers. The cold precursor is injected into the hot solvent, where the rapid temperature drop causes the seeds for the crystals to form. Thermal decomposition reactions require moderately high temperatures for the precursor to react into the required product.

A combustion synthesis route has been reported by Hwang and Wu. (71) Zinc nitrate is dispersed in glycine that acts as an organic fuel and ignited to directly form the desired nanoparticles. Various high-temperature methods are listed in Table 3.

While these methods achieve particle sizes as small as 3 nm, the reactions do not by nature fulfill the demands for green chemistry. The temperatures and solvents required are less ecologically sustainable than the low-molecular alcohols of the sol-gel methods. As for the hydrothermal methods, the particles obtained are too big or vary too much in terms of size to be considered interesting for industrial applications, other than as pigments. Even when the primary crystallite size is fairly small, as can be deduced by the peak broadening with the Scherrer equation, (88) these crystals may be aggregated to larger particles. This is evident in the products of the method as presented by Natrchalayuth et al. (73) (Fig. 2). To establish the true particle size, DLS or TEM measurements are indispensable.

A procedure that would provide small, dispersed zinc oxide nanoparticles with uniform size distribution utilizing water as solvent would be highly desirable. Unfortunately, when zinc(II)-ions react with hydroxide ions in an aqueous medium, zinc hydroxide is precipitated (1). A condensation of hydroxy groups to form zinc oxide (2) does not occur under these circumstances (68):

[Zn.sup.2+] + 2O[H.sup.-] [right arrow] Zn[(OH).sub.2] [up arrow] (1)

Zn[(OH).sub.2]([right arrow])ZnO + [H.sub.2]O (2)

This can be understood according to the principle of Le Chatelier. In a waterless environment (alcoholic solution) the condensation can occur, since the lack of water pushes the equilibrium state toward the product side. A way to bypass this obstacle is to use the zinc hydroxide as precursor for zinc oxide nanoparticles. Ghotbi published a simple, surfactant-assisted synthesis for nanoparticulate zinc hydroxide of 50-nm diameter. (83) After calcination and conversion to zinc oxide, the size is reported to diminish to 25 nm, calculated by Scherrer equation.

Such precipitation reactions offer a good opportunity for fabrication of doped materials, as the desired dopant ions are precipitated from the same solution and are thus well mixed with the main material. Ghotbi et al. (84,85) presented a synthesis for copper- and nickel-doped zinc oxide, but other metals are feasible as well.

A very fast method that does not require additional treatment and offers very small nanoparticles is the flame spray pyrolysis synthesis. The zinc salt is dissolved in a suitable fuel and sprayed into the reactor, where combustion under controlled condition leads to the formation of the desired product.

Industrial production of zinc oxide nanoparticles is not limited to a single reaction type. Patented synthetic routes for zinc oxide include, among others, hydrous and alcoholic sol-gel reactions, (89,90) mechanochcmical reactions (91) and gas-phase synthesis. (92) These methods have varying advantages and limitations, thus an evaluation of requirements must be made for any application before a decision is made. Mechanochemical and solution-based methods produce wastewater or organic waste, while gas-phase reactions offer only very limited size control but provide excellent atom efficiency.

Synthesis methods for titanium dioxide

The second important inorganic UV filter, titanium dioxide, is also well established as the active component for sun lotions. Titanium dioxide has three common modifications, anatase, brookite, and rutile, the latter generally being the most stable one. For small particles and at lower temperatures, the phase diagram is different, anatase being the preferred modification. (93) Properties such as the band gap (3.05-3.30 eV), photocatalytic activity, and refractive index depend upon the modification in question. (94) The absorption spectrum of titania nanoparticles is shown in Fig. 3.

Industrial production of titania pigment utilizes the flame spray pyrolysis method. Ti[Cl.sub.4] is burned in a pure oxygen flame, which avoids coloring impurities from iron or other metal ions. A common industrial product, Degussa P-25, is composed of a mixture of anatase, rutile and even some amorphous titania particles, and has a broad range of particle sizes. (95) Synthesis of dispersable nanoparticles with controlled sizes and morphology requires fine tuning of the reaction medium. Contrary to zinc oxide, titania nanoparticles can be formed in aqueous media, thus allowing for various hydrothermal methods. Table 4 shows various approaches to titania nanoparticle formation.

One interesting aspect of titania nanoparticles is their susceptibility to peptization. The addition of nitric acid or various tetraalkyl ammonia hydroxides allows aggregates to be redispersed into primary particles. (103,104,106)

Further methods for tilania nanoparticle formation including high pressure and plasma methods were already reviewed by Macwan et al. in 2011. (94) Methods of industrial importance are equally diverse, with flame spray reactions, (107) sol-gel reactions, (108-110) gas-phase reactions, (111) and hydrothermal reactions, (112) covering all titania modifications among them.

Synthesis methods for cerium(IV) oxide

With a band gap of about 3.0-3.2 eV, (113) good UV absorption, and a catalytically active surface, cerium oxide is pretty similar to zinc or titanium oxide in its properties. It crystallizes in a cubic fluorite structure and can easily be used to form solid solutions of other oxides, which will be discussed shortly. Ceria has many catalytic applications, on which an extensive review was published by Alessandro Trovarelli. (114)

Solution combustion, spray pyrolysis, and high-temperature decomposition

In most commercially available and comparably cheap reactants used for ceria synthesis--such as Ce[(N[O.sub.3]).sub.3] and Ce[Cl.sub.3]--cerium is contained as [Ce.sup.3+] ions and not as [Ce.sup.4+] ions, as it is required for Ce[O.sub.2]. In wet-chemical reactions, the oxidation is achieved through [H.sub.2][O.sub.2] or atmospheric oxygen in basic media before particles containing the cerium(IV) ions are formed. Combustion-based and pyrolytic methods eschew this for a more direct approach. Cerium-containing solutions are either mixed with an organic fuel such as glycine or urea and ignited (solution combustion) or dispersed as an aerosol and burned in an oxygen-rich environment (flame spray pyrolysis). High-temperature decomposition reactions avoid open flames. Instead, a metal-surfactant complex is thermally degraded in an organic solvent with a high boiling point (up to 300[degrees]C) to form the desired particles. Table 5 shows various synthetic approaches of the described type.

Regrettably, the high synthesis temperatures of solution combustion methods do not automatically warrant good crystallinity of the product. This can be achieved by an additional calcination step; however, at the calcination temperature, small particles are sintered simultaneously into hard aggregates of 1000 nm and more, (117, 118) if not already sintered during the hardly controllable combustion. (121)

Modifications of standard procedures include the methods of Hwang (118) and Chen et al. (120) Hwang et al. showed that for a homogeneous combustion a complete dissolution of ceria salts is not necessary as they can be directly dispersed in a suitable organic fuel such as urea, while Madler et al. (115) employed acetic acid both as fuel and solvent. Chen et al. implemented a procedure common to regular mechanochemical syntheses. (122) In order to prevent aggregation, inert salts (usually NaCl or KCl) are added to the milled reactants to further disperse them and thus reduce the probability of particles sticking to each other. This approach could be reused for what they described as salt-assisted solution combustion synthesis. Incidentally, the method is also applicable for the synthesis of Gd- or Sm-doped ceria particles.

Water-based chemical methods

As with zinc oxide and titanium(IV) oxide, cerium(IV) oxide synthesis is of commercial interest for a broad range of applications. An environmentally acceptable and cheap synthesis route is required to maintain environmental protection standards and fabricate affordable products. Water as solvent is always preferred whenever possible, since it is both cheap, readily available, and harmless. Thus, a variety of methods were published with water as the main reaction medium.

The most simple methods are precipitation reactions followed by a calcinations step. In the first reaction, a cerium(III) salt is dissolved and oxidized to cerium(IV). By addition of ammonia or other bases cerium, carbonate (123) or cerium hydroxide (124) is precipitated. Heat treatment of the precipitate yields the ceria nanopowder. The crystallinity depends on temperature and duration. Additional heat treatment can be avoided by conversion with [H.sub.2][O.sub.2], but the crystallinity remains rather low. (125) This method is known as soft solution synthesis.

Hydrothermal methods use sealed reactors at elevated temperature and pressure to achieve directly the desired product without the need for a calcinations step. The hydrothermal products have decent crystallinity, but the reaction requires a more sophisticated apparatus. This can be beneficial, as the hydrothermal route can be employed in a continuous process with only very short reaction times, as described by Hakuta et al. (126) A calcinations step can still be employed to further improve the crystallinity, (127) but the crystallite size and tendencies toward aggregation also increase. (128)

Another reaction type that yields cerium oxide nanoparticles without a calcinations step is the sonochemical synthesis. The mechanical agitation prevents particle aggregation during synthesis and allows for very mild reaction conditions.

Microemulsion syntheses of ceria nanoparticles use water in oil emulsions (W/O) of the cerium salt and the base. Upon mixture, the minuscule droplets act as microreactors and limit the size of the growing particle. While this method allows for good size control, the majority of the required solvent is an organic phase and additional surfactants are required. Table 6 shows various water-based synthesis methods for ceria nanoparticles.

Due to the complete dissolution of the ceria salts in water prior to the reaction, the described reactions offer a great option for the formation of doped nanoparticles and nanoparticulate solid solutions. Most notable are the efforts of Shuk, Greenblatt et al. who synthesized ceria particles doped with samarium, calcium, terbium, bismuth, europium, and various other metals. (136-142) The soft solution synthesis by Li et al. (125) is also capable to produce ceria particles doped with zinc oxide or calcia, while the microwave-assisted synthesis can be used for a Ce[O.sub.2]-Si[O.sub.2] composite material. (132)

Azeotropic distillation, a somewhat unusual reaction type for the formation of nanoparticles, was used by Zha et al. (129) In this reaction, cerium hydroxide is first precipitated along with a possible dopant metal hydroxide and then distilled in an alcoholic medium to facilitate the condensation reaction and to remove water from the system. A calcination step can be added to improve crystallinity.

With simple reaction setups and short reaction times, sonochemical syntheses are among the most promising routes. (144,145) A calcination step is not required and the seemingly low crystallinity is an intrinsic effect of the small particle size, as X-ray. A minor drawback is the requirement for additional reagents and surfactants like ethylene diamine (144) or urea (145) which is mitigated by their comparatively low cost. If the surfactant coating is present after drying of the particles, it can also reduce the aggregation during calcinations steps, leading to very small, disperse particles after heat treatment at up to 800[degrees]C. (147) On the contrary, even a combined application of peptizing surfactants and ultrasonic agitation after the precursor particles yields only particles in the ranee of 100-250 nm. (148)

The most simple, yet capable method reported so far to our knowledge is the soft solution synthesis published by Chen and Chang. (134) It offers very small, dispersed particles of good crystallinity with a simple setup and a reasonable reaction time without the necessity for surfactants, organic solvents, or other additives. A calcinations step is not required.

>Further methods for ceria nanoparticle synthesis include microwave-assisted synthesis in ionic liquids (149) and electrochemical synthesis. (150) These methods offer small, disperse nanoparticles of high quality, but their application is limited to the laboratory scale as ionic liquids are somewhat expensive solvents and their recycling is energy intensive. The electrochemical approach provided nanoparticles in the gram per hour range, but requires a high amount of electric energy. Both methods are therefore unlikely to be implemented on a large-scale production line. Common industrial methods are cerium nitrate-based sol-gel reactions, (151-153) but other routes, such as laser pyrolysis (154) and microemulsion synthesis (155) have been patented as well and are capable of large-scale production of particles with mean diameters below 10 nm. Larger particles can be obtained by hydrothermal reactions (156) or sol-gel methods with aging at elevated temperatures. (157)

Synthesis methods for iron oxides and hydroxides

Iron oxides and hydroxides with UV-absorbing qualities include various modifications of [Fe.sub.2][O.sub.3] and FeO(OH). While the absorption of UV irradiation is good, they share a common disadvantage of also absorbing parts of the visible spectrum, (158) thus coloring any formulation containing these particles in a reddish-brown hue. For most applications this is undesired, though in some cases it can be beneficial. The brown color of iron oxide-containing suntan lotions is associated with tanning and is considered more pleasing than the white color of formulations containing only zinc oxide or titania particles. Table 7 offers a selection of methods for iron-based UV-absorbing nanoparticles.

There are several common precursors for iron oxide nanoparticles. Iron(II) and Iron(III) oxatalates are used in both precipitation-calcination and high-temperature decomposition reactions. The size of the nanocrystals is in the 50 nm range, but the crystallites are aggregated and form brick-like structures of several pm size. (159,174) Additional oxidizing agents such as trimethylamine-A-oxide (161) are needed to form dispersed particles. Another downside is the requirement for thoroughly controlled reaction conditions, since small changes in temperature can cause the formation of other oxides such as [Fe.sub.3][O.sub.4], (175) which do not possess good UV-absorbing properties. (176) This is also a general problem for FeO(OH) nanoparticles. While iron(III) oxide-hydroxide can be easily obtained by precipitation, prolonged drying, or relatively mild heat treatment can, however, convert a high percentage of the material into UV-transmitting magnetite. (177) This is a major problem for industrial processes, and while particles smaller than 20 nm can be obtained, they consist mostly of non UV-absorbing magnetite. (178)

Among the smallest dispersed nanoparticles reported are those prepared by microemulsion synthesis. Since the particles cannot grow larger than the dispersed droplets of water they are contained within, the overall size is easily limited and has a narrow distribution. However, the emulsion must be stabilized and requires various surfactants to accomplish this. Ennas et al. reported comparable sizes by way of solgel synthesis, (170) but their particles were embedded in a silica matrix and the size is most likely a result of the dilution, not of a proper particle growth control. A more conventional sol-gel method was reported by Yamanobe et al., (168) but the resulting particle size is in the 150 nm range.

Surprisingly, the smallest reported [gamma]-[Fe.sub.2][O.sub.3] nanoparticles are obtained by a mechanochemical synthesis patented by Lu et al. (173) In a simple milling process, chlorides of Fe(II) and Fe(III) are mixed with potassium hydroxide using potassium chloride as the dispersion medium yielding average particle sizes smaller than 5 nm.

A number of combustion and pyrolysis-type reactions have been reported. Depending on the starting material and other reaction parameters such as concentration of the stock feed and pressure in the combustion area, resulting particle sizes vary by several orders of magnitude. (44,103,165-167) Low concentrations and pressures favor small particles with a low polydispersity index. Reaction times are fairly short but the combustions require high temperatures, well above 500[degrees]C. A good compromise between setup complexity, required temperature, and reaction time is achieved through hydrothermal synthesis. (170,171) The downside is the rather large size of the produced nanoparticles. Hydrothermal reactions are still interesting because of the various morphologies that can be produced on an industrial scale, ranging from nano-needles to prisms or cubes. (180)

Finally, in a method patented by Russo et al. iron(III) ions are first reduced to elemental iron, forming small nanoparticles. (181) These are then aged and oxidized to form small, dispersed maghemite spheres.

Strategies for the reduction of photocatalytic activity

Metal oxides, especially titanium dioxide and zinc oxide, are often valued for their photocatalytic activity, (182-183) making them interesting substances for various applications such as wastewater treatment (184) and reduction of air pollution. (185-186) When the materials are used in their capacity as UV filters instead, this reactivity becomes an undesired quality, as the generated reactive species can decompose other matter, including the polymer matrix the nanoparticles are embedded in references 187, 188.

To prevent photocatalytic activity, two general strategies can be applied. In the first approach, the nanoparticles are doped with other elements, creating defects that promote the deactivation of excited electrons, thus preventing the reactions on the particle surface leading to the formation of radicals.

For zinc oxide, Ga, Mn, and Co were investigated as dopants. Gallium led to an overall reduced capability for UV absorption, thus reducing its usefulness as UV filter. (74) Manganese was found to increase the photocatalytic activity, (189) but cobalt significantly reduced the production rate of radicals. (190,191) A drawback of this method is the change of color in the nanoparticles, as the defects cause the absorption of visible light.

The second strategy is the encapsulation of the nanoparticles in a chemically stable shell, thus preventing direct contact between the metal oxide and any photocatalytically decomposable substance or molecule that could be converted into a radical. For this shell, silicon dioxide seems to be a prime candidate, as it poses no toxicological threat, is chemically and physically stable, and does not absorb light in the visible range.

Silica coatings of varying thickness have been used on Ti[O.sub.2] (192-194) as well as ZnO nanoparticles, with thin shells obtained by Stober-like reactions with TEOS (195) and thicker shells through reactions with sodium silicate. (196) The nanovoid system presented by Wang et al. (192) is especially interesting, as it allows for both a safe and uncorroded polymer matrix and high photoreactivity for gaseous pollutants in the surrounding environment. The silica shell significantly reduced the decomposition of marker substances such as rhodamine B, phenol, nonae, or 4-nonylphenol, (192,195) or became a selective catalyst for certain molecules only, depending on the pore size of the silica shells. (192-193)

Summary

In summary, the described nanoparticles are great UV filters with high potential in industrial applications. Contrary to organic UV filters, they do not decompose, offering a long-term protection from UV irradiation for technical products. They are safe to employ in sun lotions with significant health benefits, preventing damage from UV-A and UV-B irradiation. A multitude of methods for nanoparticle synthesis have been published with various advantages and problems depending on the targeted application.

Due to the current trend toward sustainable and ecological production, one must consider not only the desired product properties but also the environmental impact of the reaction. The guidelines for green chemistry demand, among other things, for nontoxic chemicals, avoidance of high temperatures and organic solvents. Sol-gel reactions in aqueous media do not require very high temperatures or volatile organic solvents but may demand additional surfactants to control the reaction or stabilize the particles, thus adding additional chemicals to the reaction which are not required in the final product. Flame spray pyrolysis requires high temperatures but little more than the precursor salt and some suitable fuel. These factors must be taken into account when implementing a synthesis protocol on a large scale.

Further studies on controlled nanoparticle synthesis remain an important part of chemical nanotechnology since it is still difficult to manufacture ultrafine nanoparticles (<20 nm) without aggregation of the primary particles, good dispersion properties, and low polydispersity. These particles, however, are very interesting because they are small enough to avoid scattering of visible light and would allow for efficient UV filters, yet remain transparent to visible light.

The scientific and engineering community has merely begun to explore the possibilities of nanocomposite materials and while the achieved results are often impressive, the true potential lies in the combination of multiple types of particles to achieve greater functionality. While part of the research should be dedicated to the development of better synthesis methods for nanoparticles, the demand for multifunctional coatings requires a lot of attention to be focused on the combination of different particles and coating systems.

I. Fajzulin ([mail]). X. Zhu. M. Moller

DWI Leibniz Institut fur Interaktive Materialien, Forkenbeckstr. 50, 52074 Aachen, Germany

e-mail: fajzulin@dwi.rwth-aachen.de

DOI 10.1007/s11998-015-9683-2

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Table 1: Overview of mechanochemical
approaches to nanosized ZnO powders

Author                  Educts

Yang et al. (51)        Zn[Cl.sub.2], NaC[O.sub.3]
Tsuzuki et al. (52)     Zn[Cl.sub.2], NaC[O.sub.3]
Deng et al. (53)        Zn[(OH).sub.2] from Zn[Cl.sub.2] in N[H.sub.3]
Lu et al. (54)          ZnS[O.sub.4], KOH
McCormick et al. (55)   ZnC[O.sub.3] x 2 Zn[(OH).sub.2]
Shen et al. (57)        Zn(ac)2, [H.sub.2][C.sub.2][O.sub.4] x 2
                          [H.sub.2]O

                                 Calcination
                        Matrix   temperature    Particle
Author                  salt     ([degrees]C)   size (nm)

Yang et al. (51)        NaCl     600            50-150
Tsuzuki et al. (52)     NaCl     400            10-30
Deng et al. (53)        NaCl     600            20-150
Lu et al. (54)          KCl      70             10-80
McCormick et al. (55)   NaCl     300            15-40 (56)
Shen et al. (57)        NaCl     450            3-4

Table 2: Sol-gel methods for the synthesis
of zinc oxide nanoparticles

Author                      Zinc source

Bahnemann et al. (61)       Zn[(ac).sub.2]
Spanhel and Anderson (62)   Zn[(ac).sub.2]
Bai et al. (63)             Zn[(ac).sub.2]
Yadav et al. (64)           Zn[(N[O.sub.3]).sub.2]
Ba-Abbad et al. (65)        Zn[(ac).sub.2]

Lorenzen et al. (66)        Zn[Cl.sub.2]
Vega-Poot et al. (67)       Zn[Cl.sub.2]
Koch et al. (68)            Zn[(Cl[O.sub.4]).sub.2]
Iwasaki et al. (69)         Zn[(acac).sub.2]

Author                      Oxide source         Solvent

Bahnemann et al. (61)       NaOH                 2-Propanol
Spanhel and Anderson (62)   LiOH x [H.sub.2]O    Ethanol
Bai et al. (63)             KOH                  Methanol
Yadav et al. (64)           NaOH                 Water
Ba-Abbad et al. (65)        [H.sub.2][C.sub.2]   Ethanol/water mixture
                              [O.sub.4]
Lorenzen et al. (66)        NaOH                 Methanol
Vega-Poot et al. (67)       NaOH                 Ethanol
Koch et al. (68)            NaOH                 Methanol, isopropanol
Iwasaki et al. (69)         NaOH                 Methanol, ethanol

                            Temperature    Size (TEM)
Author                      ([degrees]C)   (nm)

Bahnemann et al. (61)       65             5
Spanhel and Anderson (62)   80             6
Bai et al. (63)             Room temp.     8
Yadav et al. (64)           Room temp.     40a
Ba-Abbad et al. (65)        65 (b)         20-40

Lorenzen et al. (66)        75             2-60
Vega-Poot et al. (67)       Room temp.     6
Koch et al. (68)            Room temp.     <3
Iwasaki et al. (69)         85             10-20

(a) Estimated by effective mass approximation

(b) Additional calcinations step at 400-600[degrees]C required

Table 3: High-temperature methods for
the synthesis of zinc oxide nanoparticles

Author                                      Zn source

Ma et al. (72)              Zn[(N[O.sub.3]).sub.2]
Natrchalayuth et al. (73)   Zn
Wei et al. (74)             Zn[(acac).sub.2]
Kshirsagar et al. (75)      Zn[(ac).sub.2]
Fu et al. (76)              ZnS[O.sub.4]
C.S.Li et al. (77)          Zn[([C.sub.18][H.sub.33][O.sub.2]).sub.2]
Choi et al. (78)            Zn[([C.sub.18][H.sub.33][O.sub.2]).sub.2]
Demir et al. (79)           Zn[(ac).sub.2]
C. Li et al. (80)           Zn[(ac).sub.2]
Cozzoli et al. (81)         Zn[Cl.sub.2]/Zn[Br.sub.2]
Cozzoli et al. (82)         Zn[(ac).sub.2]
Hwang, Wu (71)              Zn[(N[O.sub.3]).sub.2]
Ghotbi (83)                 [epsilon]-Zn[(OH).sub.2]
Ghotbi (84,85)              [Zn.sub.5][(OH).sub.8][(N[O.sub.3]).sub.2]
                              x 2 [H.sub.2]O
Xia et al. (30)             Zinc naphthenate
Madler et al. (86)          Zinc acrylate

Author                                       Solvent

Ma et al. (72)              Water/ethanol
Natrchalayuth et al. (73)   Water
Wei et al. (74)             Octadecene, oleylamine, xylene
Kshirsagar et al. (75)      TBPA, hexadecylamine
Fu et al. (76)              Water
C.S.Li et al. (77)          Octadecene, octyl ether
Choi et al. (78)            Oleylamine, oleic acid
Demir et al. (79)           m-Xylene, 1-pentanol
C. Li et al. (80)           Diethylene glycol
Cozzoli et al. (81)         Hexadecylamine, oleic acid, dioctyl ether
Cozzoli et al. (82)         Hexadecylamine, TOPO
Hwang, Wu (71)              Glycine
Ghotbi (83)                 --
Ghotbi (84,85)              --
Xia et al. (30)             Xylene
Madler et al. (86)          Methanol

                                 T
Author                      ([degrees]C)   Size (nm)   Type

Ma et al. (72)              180            150         HT
Natrchalayuth et al. (73)   120-170        50          HT
Wei et al. (74)             300            5-10        HI
Kshirsagar et al. (75)      310            6           TD
Fu et al. (76)              160            >300        HT
C.S.Li et al. (77)          320            10          TD
Choi et al. (78)            300            30          TD
Demir et al. (79)           130            <50         TD
C. Li et al. (80)           245            60-180      HI
Cozzoli et al. (81)         250            3-9         HI
Cozzoli et al. (82)         300            3-9         HI/TD
Hwang, Wu (71)              >500           35-60       CS
Ghotbi (83)                 300            25          PC
Ghotbi (84,85)              500            50-200      PC
Xia et al. (30)             ~2000          13          FSP
Madler et al. (86)          >>1000 (87)    1.5-12      FSP

CS combustion synthesis, HT hydrothermal, HI hot injection,
PC precursor calcination, TD thermal decomposition, FSP flame
spray pyrolysis

Table 4: Methods for the synthesis of titanium(IV) oxide nanoparticles

Author                  Ti source                 Type

Ohno et al. (96)        Ti (metal)                HT
Morselli et al. (97)    Ti[Cl.sub.4]              SG
Song et al. (98)        Ti[Cl.sub.4]              SG
Hedge et al. (99)       TiO[(N[O.sub.3]).sub.2]   SC
Arin et al. (100)       TIP                       MW
Uekawa et al. (101)     TIP                       SG
Oskam et al. (102)      TIP                       SG
Yang et al. (103,104)   TBO                       HT
Ito et al. (105)        TiOS[O.sub.4]             SG
Xia (30)                TIP                       FSP

Author                  Solvent

Ohno et al. (96)        [H.sub.2]O
Morselli et al. (97)    [C.sub.7][H.sub.8]O
Song et al. (98)        [C.sub.2][H.sub.5]OH
Hedge et al. (99)       [H.SUB.2]O/Glycine
Arin et al. (100)       [H.SUB.2]O
Uekawa et al. (101)     [H.sub.2]O
Oskam et al. (102)      [H.sub.2]O
Yang et al. (103,104)   [H.sub.2]O
Ito et al. (105)        [H.sub.2]O/[C.sub.2][H.sub.5]OH
Xia (30)                Xylene

Author                  T ([degrees]C)   Mod.   Size (nm)

Ohno et al. (96)        130-200          B      40
Morselli et al. (97)    70-100           A      35
Song et al. (98)        80               A      10
Hedge et al. (99)       300              A      5-7
Arin et al. (100)       100-140          A/AM   5-40
Uekawa et al. (101)     75               A/AM   9-15
Oskam et al. (102)      85-220           A      3-16
Yang et al. (103,104)   85-240           A/R    20-300
Ito et al. (105)        85               A      3-15
Xia (30)                ~2000            A/R    11

A anatase, AM amorphous, B brokite, R rutile, TBO titanium
butoxide, TIP titanium isopropoxide

Reaction types: FSP flame spray pyrolysis, HT hydrothermal
synthesis, MW microwave synthesis, SC solution combustion,
SG sol-gel synthesis

Table 5: High-temperature syntheses of ceria nanoparticles

Author                     Ce source

Xia et al. (30)            Ce(III) 2-ethyl-hexanoate
Madler et al. (115)        Ce[(ac).sub.3]
Gu et al. (116)            Ce[(oleate).sub.3]
Vallet-Regi et al. (117)   Ce[(N[O.sub.3]).sub.3]
Hwang et al. (118)         Ce[(N[H.sub.4]).sub.2][(N[O.sub.3]).sub.6]
Purohit et al. (119)       Ce[(N[O.sub.3]).sub.3]
Chen et al. (120)          Ce[(N[O.sub.3]).sub.3]
Sekar et al. (121)         [Ce.sub.2][([C.sub.2][O.sub.4]).sub.3]
                           Ce[([N.sub.2][H.sub.3]COO).sub.3]
                           [(N[H.sub.4]).sub.2]Ce[(N[O.sub.3]).sub.6]

Author                     Solvent/fuel                    Type

Xia et al. (30)            Xylene                          FSP
Madler et al. (115)        C[H.sub.3]COOH                  FSP
Gu et al. (116)            Various                         HTD
Vallet-Regi et al. (117)   [H.sub.2]O/[O.sub.2]            FSP
Hwang et al. (118)         [(N[H.sub.2]).sub.2]CO          SC
Purohit et al. (119)       [H.sub.2]O/glycine              SC
Chen et al. (120)          [H.sub.2]O/ethylene glycol      SC
Sekar et al. (121)         [H.sub.2]O/[N.sub.2][H.sub.4]   SC
                           [H.sub.2]O
                           [H.sub.2]O/various fuels

Author                     T ([degrees]C)   Size (nm)

Xia et al. (30)            ~2000            8
Madler et al. (115)        ~2000            5-12
Gu et al. (116)            180-290          5-20
Vallet-Regi et al. (117)   400              10
Hwang et al. (118)         >>500            10-25
Purohit et al. (119)       >>500            3-30
Chen et al. (120)          >110             5-15
Sekar et al. (121)         350              20-30

HTD high-temperature decomposition, SC solution
combustion, FSP flame spray pyrolysis

Table 6: Water-based techniques for ceria nanoparticle production

Author                                     Ce source

Zha et al. (129)                    Ce[(N[O.sub.3]).sub.3]
Hu et al. (130)                     Ce2[(O[O.sub.3]).sub.3]
J.-G. Li et al. (123)               Ce[(N[O.sub.3]).sub.3]
Arul et al. (131)                   Ce[(N[H.sub.4]).sub.2]
                                      [(N[O.sub.3]).sub.6]
Hassanzade-Tabrizi et al. (124)     Ce[Cl.sub.3]
Suresh et al. (128)                 Ce[(N[O.sub.3] ).sub.3]
Mohamed et al. (132)                Ce[(N[O.sub.3]).sub.3]
Liao et al. (133)                   Ce[(N[H.sub.4]).sub.2]
                                      [(N[O.sub.3]).sub.6]
Chen and Chang (134)                Ce[(N[O.sub.3]).sub.3]
Li et al. (125)                     Ce[Cl.sub.3]
Shuk, Greenblatt et al. (136-142)   Ce[(N[O.sub.3]).sub.3]
Hirano et al. (143)                 Ce[(N[H.sub.4]).sub.2]
                                      [(N[O.sub.3]).sub.6]
Hakuta et al. (126)                 Ce[(N[O.sub.3]).sub.3]
Yin et al. (144)                    Ce[(N[H.sub.4]).sub.2]
                                      [(N[O.sub.3]).sub.6]
Yu et al. (145)                     Ce[(N[H.sub.4]).sub.2]
                                      [(N[O.sub.3]).sub.6]
Masui et al. (146)                  Ce[(N[O.sub.3]).sub.3]
Feng et al. (147)                   Ce[(N[O.sub.3]).sub.3]

Author                              Type   T ([degrees]C)   Size (nm)

Zha et al. (129)                    AD     117              8-60
Hu et al. (130)                     P/C    400-1000         20-50
J.-G. Li et al. (123)               P/C    300-1000         5-65
Arul et al. (131)                   P/C    600              5
Hassanzade-Tabrizi et al. (124)     P/C    100-900          10-50
Suresh et al. (128)                 P/C    450-900          10-50
Mohamed et al. (132)                MW     160/500          8-15
Liao et al. (133)                   MW     ~100             2
Chen and Chang (134)                Sol    30-90            7-20
Li et al. (125)                     Sol    40               2_4135
Shuk, Greenblatt et al. (136-142)   HT     260              10-50
Hirano et al. (143)                 HT     150-240          3-9
Hakuta et al. (126)                 HT     250-400          20-250
Yin et al. (144)                    Sono   80               3-7
Yu et al. (145)                     Sono   80               3-7
Masui et al. (146)                  ME     RT               2-5
Feng et al. (147)                   ME     RT/800           25-50

AD azeotropic distillation, HT hydrothermal, ME microemulsion, MW
microwave, P-C precipitation-calcination, Sol soft solution,
Sono sonochemical

Table 7: Iron-based UV-absorbing nanoparticles

Author                          Method                Fe source

Suresh et al. (159)       Prec./calc.           Fe([C.sub.2][O.sub.4])
Miao et al. (160)         Precipitation         Fe[Cl.sub.3]
Calvius et al. (161)      HTD                   [Fe.sub.2][([C.sub.2]
                                                  [O.sub.4]).sub.3]
Bumajdad et al. (162)     Microemulsion         Fe[(N[O.sub.3]).sub.3]
Cabanas et al. (163)      Flame spray           Fe[(N[O.sub.3]).sub.3]
                            pyrolysis           Fe([C.sub.6][H.sub.5]
                                                  [O.sub.7])
Grimm et al. (164)        Flame spray           Fe(acac)
                            pyrolysis           Fe[(CO).sub.5]
Janzen and Roth (165)     Gas combustion        Fe[(CO).sub.5]
Yi et al. (166)           Gas combustion        Fe[([C.sub.5][H.sub.7]
                                                  [O.sub.2]).sub.3]
Suresh and Patil (167)    Solution combustion   Fe[(NO).sub.3]
Yamanobe et al. (168)     Sol-gel               Fe[(N[O.sub.3]).sub.3]
Makie et al. (169)        Sol-gel               Fe[(N[O.sub.3]).sub.3]
Ahmmad et al. (170)       Flydrothermal         Fe[(N[O.sub.3]).sub.3]
Hua and Gengsheng (171)   Hydrothermal          Fe[Cl.sub.3]
Vollath et al. (172)      Microwave             Fe[Cl.sub.3]
Lu et al. (173)           Mechanochem           Fe[Cl.sub.3]/
                                                  Fe[Cl.sub.2]

Author                    Size (nm)          Modification

Suresh et al. (159)       60 (a)      [alpha]/[gamma]-
                                        [Fe.sub.2][O.sub.3]
Miao et al. (160)         5-50        [alpha]-FeO(OH)
                                      [gamma]-[Fe.sub.2][O.sub.3]
Calvius et al. (161)      10-50 (a)   [alpha]-[Fe.sub.2][O.sub.3]
Bumajdad et al. (162)     4-10        [alpha]-[Fe.sub.2][O.sub.3]
                                      FeO(OH)
Cabanas et al. (163)      200-2000    [alpha]/[gamma]-
                                        [Fe.sub.2][O.sub.3]
Grimm et al. (164)        5-25        [gamma]-[Fe.sub.2][O.sub.3]
Janzen and Roth (165)     4-20        [gamma]-[Fe.sub.2][O.sub.3]
Yi et al. (166)           3-5         [alpha]-[Fe.sub.2][O.sub.3]
                          20          [gamma]-[Fe.sub.2][O.sub.3]
Suresh and Patil (167)    200         [gamma]-[Fe.sub.2][O.sub.3]
Yamanobe et al. (168)     120-180     [alpha]/[gamma]-
                                        [Fe.sub.2][O.sub.3]
Makie et al. (169)        30          [alpha]-FeO(OH)
Ahmmad et al. (170)       40-80       [alpha]-[Fe.sub.2][O.sub.3]
Hua and Gengsheng (171)   70-100      [alpha]-[Fe.sub.2][O.sub.3]
Vollath et al. (172)      4-7         [gamma]-[Fe.sub.2][O.sub.3]
Lu et al. (173)           0.5-10      [gamma]-[Fe.sub.2][O.sub.3]

(a) Size of primary crystallites determined by Scherrer equation


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Author:Fajzulin, Igor; Zhu, Xiaomin; Moller, Martin
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Jul 1, 2015
Words:13654
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